1. Field of the Invention
The present invention relates generally to improvements in optical communication systems. More particularly, the present invention relates to chromatic dispersion compensation in optical communication systems which utilize optical phase conjugation or other types of optical signal conversion.
2. Description of the Prior Art
In optical communication systems which utilize optical fiber as a transmission medium, chromatic dispersion and fiber nonlinearities present significant obstacles to achieving higher system data rates and longer repeaterless transmission distances. Chromatic dispersion, often simply called dispersion, refers to a phenomenon in which the speed of an optical signal through an optical transmission medium such as fiber varies as a function of the optical signal wavelength. The problem of chromatic dispersion is particularly significant in the standard single-mode fiber (SMF) making up much of the world's existing optical fiber infrastructure. Standard SMF typically exhibits a dispersion zero at a wavelength of about 1300 nm, with positive dispersion for wavelengths longer than the dispersion zero.
Dispersion can be expressed in terms of variations in the propagation constant of the fiber with respect to frequency. First- and second-order group velocity dispersion refer to the second and third derivatives of the fiber propagation constant .beta. with respect to angular frequency .omega., or .beta..sub.2 and .beta..sub.3, respectively. Higher order dispersion terms can be approximated as zero in most applications. When used in the context of lightwave transmission systems, first- and second-order dispersion are commonly expressed in terms of derivatives with respect to wavelength. Thus, first-order group velocity dispersion is typically expressed as a change in pulse propagation time over a unit length of fiber with respect to a change in pulse wavelength. In this case, the symbol D(.lambda.) is often used to refer to first-order group velocity dispersion, and the units are typically picoseconds per nanometer-kilometer (ps/nm-km). Second-order group velocity dispersion is then expressed, using units of .lambda.ps/nm.sup.2 -km, as the derivative with respect to wavelength of D(.lambda.).
An important fiber nonlinearity that can limit transmission capability is the Kerr effect, in which the index of refraction increases with the intensity of an applied optical signal. Changes in the fiber index of refraction modulate the phase of an optical signal passing through the fiber, and thereby redistribute the signal frequency spectrum. In systems in which the optical signal modulates itself, this phenomenon is generally known as self-phase modulation. Self-phase modulation creates lower frequencies toward the leading edge of an optical signal pulse and creates higher frequencies toward the trailing edge. In multi-channel systems, in which one signal causes modulation of other signals, the phenomenon manifests itself as either cross-phase modulation or four-photon mixing. For both single-channel and multi-channel systems, the resulting changes in frequency distribution are translated to amplitude modulation by the fiber dispersion. The interplay between chromatic dispersion and nonlinearities such as the Kerr effect therefore can lead to increasing optical signal distortion as a function of transmission distance. For long distance communication over optical fiber, therefore, dispersion and nonlinearities must be controlled, compensated or suppressed.
Midspan optical phase conjugation is a technique which has been used to alleviate the effects of chromatic dispersion in optical systems. Because the phase conjugate of an optical pulse is, in effect, a time reversal of the pulse, an optical phase conjugator placed at the midpoint of an optical fiber span allows the first-order group velocity dispersion of the first half of the span to be compensated by the identical first-order group velocity dispersion produced as the conjugated signal propagates along the second half of the span. See, for example, A. Gnauck, R. Jopson and R. Derosier, "10 Gbit/s 360 km Transmission over Dispersive Fiber Using Midsystem Spectral Inversion," IEEE Photonics Technology Letters, Vol. 5, No. 6, pp. 663-666, June 1993; and S. Watanabe et al., "Compensation of Chromatic Dispersion in a Single-mode Fiber by Optical Phase Conjugation," IEEE Photonic Technology Letters, Vol. 5, No. 1, pp. 92-95, January 1993. Dispersion compensation using optical phase conjugation has also been demonstrated for wavelength division multiplexed (WDM) systems. See, for example, A. Gnauck, R. Jopson, P. Iannone, and R. Derosier, "Transmission of two wavelength-multiplexed 10 Gbit/s channels over 560 km of dispersive fibre," Electronics Letters, Vol 30 No. 9, pp. 727-728, April 1994.
U.S. patent application Ser. No. 08/120 014 (now U.S. Pat. No. 5,365,362) entitled "Ultra-High Capacity Non-Soliton Optical Transmission Using Optical Phase Conjugation," assigned to the assignee of the present invention and incorporated herein by reference, first recognized that the effects of fiber span nonlinearities could be compensated using optical phase conjugation. An exemplary optical system described in U.S. Pat. No. 5,365,362 adjusts optical signal power in a fiber span by selecting appropriate number, spacings and output power for in-line amplifiers such that the effects of fiber non-linearities are compensated. Further detail regarding compensation of fiber nonlinearities using optical phase conjugation may be found in, for example, C. Kurtzke and A. Gnauck, "How to Increase Capacity beyond 200 Tbit/s-km without Solitons," ECOC '94 Proceedings, Vol. 3, Postdeadline Paper No. ThC 12.12, pp. 45-48, Montreux, Switzerland, September 1993; W. Pieper et al., "Nonlinearity-insensitive standard-fibre transmission based on optical-phase conjugation in a semiconductor-laser amplifier," Electronics Letters, Vol. 30, No. 9, pp. 724-726, 1994; and S. Watanabe and T. Chikama, "Cancellation of four-wave mixing in multichannel fibre transmission by midway optical phase conjugation," Electronics Letters, Vol. 30, No. 14, pp. 1156-1157, July 1994.
An additional concern which may arise in the optical systems described in U.S. Pat. No. 5,365,362 is the effect of chromatic dispersion introduced by the optical phase conjugator itself. For example, a common technique for phase conjugating an optical signal is four-photon mixing in a length of dispersion-shifted fiber (DSF). Efficient mixing in the DSF generally requires proper phase matching, which may be achieved by placing a pump signal wavelength near the dispersion zero of the DSF or, if two pump signals are used, by placing the average of their wavelengths near the dispersion zero. It has been discovered, however, that under these conditions chromatic dispersion in the DSF phase conjugator can significantly distort the phase conjugated output signal. In a phase conjugator consisting of 20 km of DSF with a dispersion zero at 1550 nm, a pump signal at a wavelength of 1550 nm and an input signal at a wavelength of 1554 nm, a phase conjugated output signal will be generated at a wavelength of approximately 1546 nm. If the DSF has a typical second-order dispersion value of 0.08 ps/nm.sup.2 -km, the DSF will produce a total first-order dispersion at the wavelength of the conjugated signal of (1546 nm-1550 nm).times.0.08 ps/nm.sup.2 -km, or 0.32 ps/nm-km. The conjugated signal at the output of the DSF will therefore exhibit a substantial additional chromatic dispersion of (-0.32 ps/nm-km).times.(20 km), or -6.4 ps/nm. The chromatic dispersion produced within an optical phase conjugator can thus be sufficiently large to reduce or eliminate any benefit obtained from the phase conjugation. Similar concerns apply with other types of optical signal convertors utilizing DSF, single-mode fiber (SMF) or other alternative nonlinear conversion media.
As is apparent from the above, a need exists for an optical phase conjugator, frequency shifter or other signal convertor which includes compensation for chromatic dispersion introduced as a result of the conversion.